Azeotropic mixtures or close boiling mixtures have been conventionally separated by large scale multistage distillations or, sometimes, by combined processes such as extractive distillation. These separation processes are generally characterized by high energy demands, relatively large capital plant investments, a variety of maintenance problems, and severe environmental pollution problems whose solution contribute to the energy usage.
Attention has recently been focused on mixtures produced as by-products of the chemical and petroleum industries. Separation of these mixtures into their components has frequently been difficult and consequently certain of the by-products have been deemed as expendable and disposable. The development of a low cost technique for separating such by-product mixtures would clearly be of great benefit.
Conventional separation techniques such as distillation, adsorption, liquid-liquid extraction and crystallization are often insufficient and uneconomical. Application of separation processes which involve the use of porous and/or semi-permeable membranes for separating compounds can save in process costs because energy consumption is low, raw materials and nutrients can be recovered and reused, fermentation processes can be carried out continuously, and disposal problems can be reduced or eliminated. The separation processes using membrane technology include: ultrafiltration, reverse osmosis, pervaporation and electrodialysis, in combination with distillation.
Ultrafiltration is employed to separate colloidal particles according to their particle size (preferably, 10 microns to 10 nanometers). The process involves feeding a liquid mixture through a microporous membrane while both sides of the membrane are maintained at high operating pressure, e.g., 7 bars. Reverse osmosis involves feeding a liquid mixture on one side of a membrane at high operating pressures, e.g., 7 bars, while maintaining the system on the opposite side of the membrane at atmospheric pressure. Thus, the resulting permeate remains in the liquid phase. Suitable conventional reverse osmosis membranes consist of cellulose derivatives. A disadvantage of the reverse osmosis and ultrafiltration process employing conventional membranes is that the highest concentration of the liquid mixture that can be obtained is about 20% due to the high osmotic pressure requirements.
The pervaporation process involves feeding a liquid mixture on one side of a membrane at atmosphere pressure, while maintaining the system on the opposite side of the membrane at a sufficiently low vapor pressure to vaporize one liquid component by employing a vacuum pump or an inert gas flow. The resulting component (permeate) that is permeated through the membrane is vaporized and is collected in a gaseous state. The advantages of this process is that it can be applicable to the separation of azeotropic mixture that cannot be separated by an ordinary distillation, to the separation of a mixture of compounds having close boiling points, to the concentration of a compound which is sensitive to heat, or to the separation of isomers. Moreover, unlike reverse osmosis, these separations or concentrations are applicable over the entire range that is to be treated.
For the pervaporation processes, the efficiency of the membrane is evaluated by the permselectivity which is measured by the permeability flow rate (flux) and the separation factor (selectivity). The separation factor (S. F.) is defined as the ratio of the concentration of two substances A and B in the permeate divided by the ratio of the concentration of the same substances in the feed. Specifically, for separation of water-alcohol mixtures, the separation factor (S. F.) is calculated according to the following equation: EQU S. F. =(Yw/Yal)/(Xw/Xal)
where:
Yw: concentration of water in the permeate PA1 Yal: concentration of alcohol in the permeate PA1 Xw: concentration of water in the feed PA1 Xal: concentration of alcohol in the feed PA1 n has a value of 1.0-5.0; PA1 x has a value of about 0.2 to about 0.7; PA1 y has a value of 1 minus the value of x; PA1 R is ##STR5## R.sup.1 is ##STR6## R.sup.2 is a C.sub.1 -C.sub.5 lower alkylene group; V is a vinyl group; and PA1 Me is a methyl group. PA1 (a) dissolving the organosiloxane block copolymer and the linking reagent in a solvent to form a first solution; PA1 (b) adding a catalytically effective amount of a hydrosilylation catalyst to the first solution to form a second solution; PA1 (c) coating the hydrophilic asymmetric membrane with the second solution; PA1 (d) allowing the coated composite membrane to swell; and PA1 (e) drying the swollen coated composite membrane.
Two types of membranes have been conventionally employed for the separation of water-alcohol mixtures in the pervaporation process: water-permselective membranes and alcohol-permselective membranes. For alcohol dehydration, water-permselective membranes are employed and the permeate is water. Thus, water is in the saturating vapor phase. Processes using conventional water-permselective membranes employ mixtures containing up to about 10% water. In contrast, when alcohol-permselective membranes are employed, the permeate is alcohol. Thus, alcohol is in the saturating vapor phase. Processes using conventional alcohol-permselective membranes employ mixtures containing up to about 10% alcohol.
Alcohol-permselective membranes are conventionally hydrophobic. U.S. Pat. No. 4,591,440 to Higashimura describes one such membrane, a substituted acetylene polymer membrane. Higashimura's data shows that, in the process of evaporating mixtures having a water concentration of 90% through his substituted acetylene polymer membranes, the separation factor equals 0.033 and the permeability flow rate equals 1.91 kg/h.m.sup.2. In addition, Higashimura describes a method of producing his membranes which is time consuming and inefficient. Kimura et al., 8(3) Membrane 177-83 (1983) describes a hydrophobic membrane made of silicone rubber. Kimura's data shows that, in the process of evaporating mixtures with water concentrations of 90% through silicone rubber membranes, the permeability flow rate equals 0.140 Kg/h.m.sup.2.
Comparative Example 2 of U.S. Pat. No. 4,591,440 describes a composite membrane consisting of a hydrophobic coating grafted on a chemically inert membrane, e.g.. polytetra-fluoroethylene ("PTFE"). This composite membrane is prepared by applying a silicone rubber coating (500 micrometer thick) on a PTFE base and then exposing it to U. V. light at room temperature to obtain a uniform film of approximately 70 micrometers thick. In the process of evaporating water-ethanol mixtures having a water content of 90% and with the vapor phase saturated with ethanol, the separation factor equals 0.14 and the permeability equals 0.067 kg/h.m.sup.2.
As the above references demonstrate, the conventional alcohol-permselective membranes have numerous disadvantages. One disadvantage is the small operating range for membrane concentrating efficiency (up to 15% of alcohol). Another disadvantage is the low alcohol permselectivity. Further, as the alcohol concentration in the feed rises above 15%, the alcohol permselectivity through the membrane decreases substantially.
Conventionally, the art teaches that water-permselective membranes are hydrophilic membranes in order to preferentially attract the water. See Toshihiro et al., 36 Journal of Applied Polymer Science 1717 (1988). Examples of conventional hydrophilic membranes are polyalcohols, polyamides, polyethers and polyesters. Aptel et al., 16 Journal of Applied Polymer Science 1061 (1972) describe a water permselective membrane made from a hydrophilic membrane of cellulose acetate. When evaporating a water-ethanol mixtures having a water content of 4%, Aptel's data shows a separation factor equaling 3.0 and a permeability flow rate equaling 0.29 kg/h.m.sup.2.
Hirotsu et al., 36 Journal of Applied Polymer Science 717 (1988) describes a hydrophilic membrane composed of polyvinyl alcohol crosslinked by fluorescent light. When evaporating a water-ethanol mixture having a water content of 10% through such membranes, Hirotsu's data shows a separation factor equaling 21 but a permeability flow rate equaling only 0.02 kg/h.m.sup.2. Moreover, for evaporating mixtures having a water content of 0.5% through such membranes, Hirotsu's data shows a separation factor equaling 170 but a permeability flow rate dropping to zero.
Hirotsu et al., 6(12) Research Institute Polymer Text, Japan, 33 (1986) describe a hydrophilic composite membrane prepared by coating a photocrosslinkable acrylic acid on a porous polypropylene film. When evaporating water-ethanol mixture having a water content of 10% through such membrane, Hirotsu's data shows a separation factor equaling 36 and a permeability flow rate equaling 0.1 kg/h.m.sup.2.
U.S. Pat. No. 4,728,429 to Cabasso et al. describes a method of preparing a hydrophilic membrane from a sulfonated ion-exchange polyalkylene. When evaporating a water-ethanol mixture having a water content of 14.6% through such a membrane, Cabasso's data shows a separation factor equaling 725 and a permeability flow rate equaling 0.152 kg/h.m.sup.2.
As the above references illustrate, conventional water-permselective membranes have numerous disadvantages. One is, as the concentration of alcohol in the feed increases, the permeability flow rate through the membrane substantially decreases. That decrease is believed to be characteristic for all conventional water permselective membranes because, as the liquid passes through the membrane, the membrane swells. Thus, when the water content decreases below 10%, the permeability rate decreases substantially. As a result, such membranes generally cannot be used to effectively remove all the water from an alcohol-water mixture. Therefore, these membranes cannot effectively be used for alcohol dehydration.